My experience and my success tells me you gotta build something of value and as MPBiz said ou gotta persistent
Now what you build doesn't have to be complex, an invention and it can be a trick -and as Larry Ellison said - Google is one trick pony but it's a damned good trick! -but you have to build enterprise value to build sustainable scale and wealth
And one thing creates EV: customers who like what you do
G! Has lots of customers that buy and buy and buy
And g! Has leveraged the success of that trik to build a huge sales force, buy DCLK, buy mobile ad NTSC, buy 50 small co a year and fund Android and oh yeah Motorola - creator of the WF loved Startac

and even they miss - recall Google audio ?
I think the quote is build something useful and the world will beat a path to your door
If you are still thinking Adsense SEO and 1000s of sites is a winning scheme - based on your own lack of success and the advice here - then I would pack it in
If you think hard about creating something that customers will want and focusing and busting yournass to build something then awesome - and it can be opening a TCBY franchise or a website or anything - but if you want stability build something that will be stable b/c customers will come back and back back for more of what you do
Good luck